Relation Between Middle Cerebral Artery Blood Flow Velocity and Stump Pressure During Carotid Endarterectomy

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1 1439 Reltion Between Middle Cerebrl Artery Blood Flow Velocity nd Stump Pressure During Crotid Endrterectomy Merrill P. Spencer, MD; George I. Thoms, MD; nd Mrk A. Moehring, MSEE Bckground nd Purpose: Mny ptient monitoring techniques hve been used for detecting cerebrl hypoperfusion during crotid endrterectomy. We compred middle cerebrl rtery blood flow velocities with crotid rtery stump pressures to evlute the indictions for common crotid rtery cross-clmp shunting nd the probble hemodynmic cuses of cerebrovsculr complictions. Methods: Bloodflowvelocities were monitored with trnscrnil Doppler ultrsound nd crotid stump pressures were mesured t the time of common crotid rtery cross-clmping during 97 crotid endrterectomy procedures. Stump pressures mesured with the guge zero reference t the common crotid rtery level were correlted with the percentge chnge of velocities. Results: Middle cerebrl rtery blood flow velocities usully decresed upon common crotid rtery cross-clmping, depending on collterl vilbility nd the utoregultion response. The best fit of the dt ws to n exponentil function concve to the pressure xis, with velocity s percentge of the pre-cross-clmp vlue reching zero t 15 mm Hg stump pressure (r=0.85 ndp<0.001). Conclusions: There is less criticl mrgin of error with percentge middle cerebrl rtery blood flow velocity decreses thn with stump pressure mesurements. This reltion estblishes chnges in middle cerebrl rtery blood flow velocities s relible prmeter for judging the effects of crotid crossclmping on cerebrl blood flow nd providing n excellent indictor s to the necessity for shunting. (Stroke 1992;23: ) KEY WORDS bloodflowvelocity crotid endrterectomy ultrsonics Surgicl opertions requiring cross-clmping of the crotid rtery threten the cerebrl circultion nd the brin if regionl hypoperfusion, emboliztion, or hyperperfusion t clmp relese re produced. If, through improved monitoring, these threts cn be reduced, then crotid endrterectomy nd other opertions ffecting cerebrl perfusion my become more universlly ccepted with extended clinicl indictions. Vrious modlities used to monitor nd determine the vulnerbility of the brin during crotid cross-clmping hve included: 1) mesurement of the distl internl crotid rtery (ICA) pressure response upon common crotid rtery (CCA) clmping (i.e., stump pressure), ) ptient response monitoring under locl nesthesi, 3) electroencephlogrphic (EEG) monitoring, nd 4) jugulr venous oxygen sturtion. 14 Jugulr venous oxygen sturtion chnges represent more territory thn tht perfused by the ICA nd From the Centers for Stroke Reserch (M.P.S.) nd Bioengineering (M.A.M.), Institute of Applied Physiology nd Medicine (G.I.T.), Spentech Inc. (M.P.S.), nd Providence Medicl Center (G.I.T.), Settle, Wsh. Supported by privte funds of the Institute of Applied Physiology nd Medicine nd Spentech Inc. No governmentl support ws used. Address for correspondence: Merrill P. Spencer, MD, Institute of Applied Physiology nd Medicine, th Avenue, Settle, WA Received Jnury 28,1992; finl revision received My 28,1992; ccepted My 29, thus my underestimte chnges in oxygen extrction of the thretened territory. Serious reduction in corticl perfusion is more directly indicted by EEG or ptient response monitoring nd is widely used tody for tht reson. Operting on conscious ptient, however, sometimes increses the technicl difficulty of the opertion, nd dverse EEG chnges or ptient responses re often somewht delyed compred with the responses of stump pressure or middle cerebrl rtery (MCA) blood flow velocity. Stump pressure mesurements represent the thretened territory but do not recognize emboliztion nd my be incorrect becuse of inconsistent zero referencing of the pressure guge. Recently trnscrnil Doppler ultrsonogrphy (TCD) hs evolved s method of monitoring blood flow velocities in the MCA of ptients undergoing crotid surgery, being first reported in Through series of subsequent reports compring TCD with EEG nd stump pressures the bility of TCD to ssess hemodynmic nd embolic phenomen developed. Additionl disclosures of the vlue of TCD monitoring during crotid endrterectomy hve dded to our knowledge. TCD provides both hemodynmic nd embolic informtion relting to perfusion of the thretened territory. The purpose of this report is to demonstrte the reltion between MCA blood flow velocity chnges nd the crotid bck (stump) pressures found immeditely following cross-clmping of the CCA nd to relte this informtion to previous studies on stump pressure,

2 1440 Stroke Vol 23, No 10 October 1992 A B c (\ A A ; CCA PRESSURE & ; MCA VELOCITY FIGURE 1. Trcings of common crotid rtery (CCA) pressure (upper trcing) nd middle cerebrl rtery (MCA) Doppler blood flow velocity spectrum (lower trcing). A, immeditely before cross-clmping CCA; B, during first three pulses fter cross-clmping; C, during second intervl fter cross-clmping. Note tht between B nd C velocities increse long with decrese in pressure. MCA blood flow velocities, nd regionl cerebrl blood flow (rcbf) regrding indictions for selective shunting nd to the cuses of cerebrovsculr complictions from the surgery. Subjects nd Methods A 2-MHz pulsed Doppler ultrsound device ws used to ccess the MCA or one of its brnches through the trnstemporl pproch during cross-clmping of the CCA in crotid endrterectomy opertions. The length of the ultrsonic smple volume ws 12 mm, centered t depth of either 4.5 or 5 cm from the crystl fce. A depth of 4.5 cm ws preferred nd used in 77% of the cses to void the overlpping of collterl effects from the nterior cerebrl rtery or posterior cerebrl rtery sometimes occurring t depth of 5 cm. The verge MCA blood flow velocity (vmca) recorded t the two depths ws identicl t 38 cm/sec. The mximum ultrsound power used ws 900 mw/cm 2, djusted downwrd when good signls were obtined, to produce n udiotpe record level of -5 to -10 db. A specilly designed hedbnd (Trnspect, MedSonics, Fremont, Clif.) fixed the probe to the ptient's hed with provisions for djusting the probe position nd ngle. When MCA velocities dropped to immesurble levels, below 5 cm/sec (i.e., below the equipment's high-pss cutoff frequency of 200 Hz), the systolic nd distolic velocities were entered s 1 cm/sec to void mbiguities in the dt bse formuls. For the bseline vlues, velocities nd pressures were verged over one hlf of respirtory cycle before cross-clmping. Men velocity for the first bet fter cross-clmping ws mesured if rpid clmping took plce. The velocities nd pressures during second intervl fter cross-clmping were lso mesured. Figure 1 illustrtes the three periods of mesurement. Assurnce tht the monitoring ws performed on the MCA or brnch thereof ws usully confirmed by oscilltions of the crotid rtery incurred spontneously during dissection or upon deliberte finger oscilltions of the crotid rteries by the surgeon. The oscilltions were trnsmitted in the blood column to the MCA nd were visible on the Doppler spectrum to CAROTID-REFERENCE STUMP PRESSURE FIGURE 2. Sctter plot of differences between stump pressures mesured with guge zero reference t level of common crotid rtery nd those mesured with guge zero estimted t hert level, x, line of identity; vlues in mm Hg. The stump pressure ws mesured, by mens of strin guge mnometer connected through sline-filled tubing to needle puncturing the CCA, t the time of cross-clmping of the CCA nd fter cross-clmping of the externl crotid rtery nd its immedite brnches. The 97 cses reported here were selected from lrger dt bse using only those tht hd ccurte stump pressures mesured with method for consistent guge zero referencing. The zero reference level of the pressure mesurement ws set t the CCA nd ws indicted utomticlly t the moment just prior to insertion of the needle into the rtery on the nlog trcing of strip chrt recorder. This technique ssured n ccurte nd consistent zero reference nd documented pressure mesurements for lter review. In 85 of the 97 cses the routine stump pressure reported by the nesthesiologist used guge zero reference level vribly estimted by the level of the hert. These mesurements were not used in our quntittive nlysis becuse of the uncertinty of the zero reference. When both mesurements were compred (Figure 2), the hert-referenced mesurements verged 6.4 mm higher nd one third exceeded the CCA-referenced mesurements by >10 mm Hg. The stump pressures reported herefter re only those mesured from the strip chrt recordings using the CCA s the guge zero reference. A resistnce index ws clculted s men pressure divided by men velocity to correct for chnges in velocity incident to simultneous chnges in pressure. All signls, including the Doppler spectrum, stereoseprted directionl Doppler signls, voice nnottions, nd rteril pressures, were recorded on four-chnnel udio/video recorder. Arteril pressures were recorded on one udio chnnel by mens of voltge-controlled oscilltor in ddition to the strip chrt recordings. By this mens, the dt could be replyed nd vlidted fter the opertion to ssure simultneity nd ccurcy. Systolic nd distolic velocities nd pressures were entered into n nlytic dt bse (REFLEX), nd men vlues clculted s 0.4x(systolic-distolic)+distolic. The fctor 0.4 ws determined by tking smple of dt from our ptient popultion nd estimting, with cursor, the true men velocity nd men pressure just before cross-

3 Spencer et l MCA Blood Velocity nd Stump Pressure 1441 E «o > o s 60 on * DO n 1 * h SO 100 ICO CCA PRESSURE (mmhg) FIGURE 3. Sctter plot of reltion between common crotid rtery (CCA) pressures nd middle cerebrl rtery (MCA) blood flow velocities before (+) nd seconds fter () cross-clmping, x, best fit of exponentil function with T=0.64. clmping. We found 0.38 to be the verge fctor tht clcultes men velocity from systolic nd distolic vlues. Similrly, the fctor for pressure ws Using 0.40 insted of these vlues, we overestimte men velocity by 1 cm/sec nd men pressure by 1 mm Hg. Since we re compring velocity percentge with pressure, the error is inconsequentil. The dt were obtined from the opertions of six surgeons who used selective shunting bsed on the nesthesiologist's stump pressures s well s MCA velocities. Cross-clmp times were recorded in minutes s single number if not shunted nd s two numbers representing the times required for insertion nd removl if shunted. Microemboli detected in the nesthesi recovery room were recorded s number per monitoring minutes. Anesthesi routinely consisted of induction with thiopentl nd mintennce with N 2 O in 0 2 combined with either enflurne or isoflurne. Crbon dioxide tension in the rteril blood ws held slightly hypocpnic. Cerebrovsculr complictions while hospitlized were grded on scle of 1 to 5, with 1 representing trnsient ischemic ttck, 2 persistent deficit not limiting ordinry ctivity, 3 deficit limiting ctivity but ptient ble to cre for self, 4 ptient needing help with personl cre, confined to wheelchir, or wlking with ssistnce, nd 5 bedridden ptient. K-N Results Immeditely upon cross-clmping, MCA velocities usully diminished (Figure 3). First-bet velocities v SO c d. r HI < 1» 20 D D * " 0 10 ZO CCA STUMP PRESSURE (mmhg) FIGURE 4. Sctter plot of reltion between middle cerebrl rtery (MCA) men blood flow velocity s percentge of vlue before cross-clmping nd men common crotid rtery (CCA) stump pressure, x, best fit of exponentil function with T=0.85. erged 53 ±24% (men±sd) of the bseline vlues, rnging from 100% to 3%. After the first bet velocities incresed, on verge 4 cm/sec towrd bseline vlues, when mesured t the second intervl. The vrition in return depended on blood pressure, which during this phse usully diminished nd verged 6 mm Hg lower thn t the first bet. The resistnce index clculted t the first bet nd t the second intervl were compred. In 65 ptients in whom both sets of dt were vilble nd velocities did not drop below 5 cm/sec, there ws decrese in resistnce fter cross-clmping verging from 2.5 ±1.0 to 1.8 ±0.6 mm Hg/cm/sec. This implied vsodilttion is ssumed to represent utoregultion nd occurred in ll but two ptients whose resistnce index did not chnge fter cross-clmping. The combined reltion between vmca both before nd fter cross-clmping nd stump pressure (p) both before nd fter cross-clmping fit n ellipticl function with r=0.63 nd/?<0.001; the pressure intercept of this velocity function ws 20 mm Hg. The reltion between MCA velocity fter cross-clmping clculted s percentge of the bseline vlue (vmca%) nd stump pressure lso fit n ellipticl function with r=0.82 nd /?<0.001; the pressure xis intercept of this function ws 19 mm Hg. Both reltions lso fit exponentil functions with slightly better correltion coefficients: vmca=40 [l-e- O35(p - 9) ], r=0.64, /><0.001 (Figure 3) nd vmca% = 100[l-e- 055(p - 15) ], /-=0.85, /7<0.001 (Figure 4). The pressure xis intercepts of these functions were 9 nd 15 mm Hg, respectively. Compring the reltion c TABLE 1. Contrlterl ICA Occlusion nd Cerebrl Hemodynmics in 90 Ptients With Crotid Endrterectomy Prmeter Stump pressure (mm Hg) Blood flow velocity (% bseline) O Averge P Mximum O P Minimum O P Stndrd devition O P ICA, internl crotid rtery; O, 12 ptients with occlusion of contrlterl ICA; P, 78 ptients with ptent contrlterl ICA.

4 1442 Stroke Vol 23, No 10 October 1992 TABLL 2. #pt Sx T D T A T A D Cerebrovsculr Complictions in Seven Ptients With Crotid Endrterectomy ICA stenosis (%) Homolterl Contrlter l vmca% Stump pressure (mm Hg) Shunt N/Y No No Yes Yes N/Y N/Y Clmp time (min) Microemboli (no./min) 44/45 14/18 0/1 0/1 7/2 0/5 97/37 Compliction grde Cuse E E+HT E in OR Hypo Hyper+E+Hypo Hypo+E Hypo+E Computed tomogrm ICA, internl crotid rtery; Sx, preopertive symptom; T, trnsient ischemic ttck; D, dizziness; A, symptomtic; vmca%, middle cerebrl rtery blood flow velocity fter cross-clmping s percent of bseline; no./min, number of microemboli detected in recovery room/minutes monitored; N/Y, prolonged insertion or removl time; E, embolus; HT, hypertensive episode in recovery room; OR, operting room; Hypo, hypoperfusion; Hyper, hyperperfusion; *no chnge from preopertive trophy; toccipitoprietl infrct; themorrhgic infrcts in both hemispheres with diffuse cerebrl edem; left frontoprietl infrct. * t * between vmca% nd percentge pressure chnge, the correltion coefficient for n exponentil function ws There were 17 ptients with <10% decrese in vmca upon cross-clmping. Among the 13 who underwent preopertive ngiogrphy, 10 hd >89% stenosis nd in four of these "slow flow" or "string sign" ws demonstrted. Five with stenosis of >50% pprently hd very efficient collterl systems. However, tight lesions (>95% re stenosis) did not predict high cross-clmp vmca%; rther, ptients with >90% stenosis were well distributed over the rnge of vmca%. The lowest stump pressure recorded ws 8 mm Hg; this ptient ws one of five with stump pressures verging 19 (rnge 8-25) mm Hg in whom vmca dropped below 5 cm/sec. This ptient demonstrted preopertive contrlterl occlusion of the ICA. However, contrlterl occlusion did not predict routinely lower stump pressures or routinely lower cross-clmp velocities, but ptients with contrlterl occlusion demonstrted lower verge stump pressures nd velocities (Tble 1). Cerebrl complictions were observed in the immeditely postopertive period in seven ptients, two mong the 64 nonshunted nd five mong the 33 shunted surgeries. Tble 2 lists ll ptients with complictions in decresing order of vmca% nd indictes the probble cuse or cuses of the compliction concluded from the vilble dt. None of the seven demonstrted postopertive occlusion of the ICA. The complictions in ptients 200, 211, nd 55 were concluded to be cused by emboliztion becuse 25 other nonshunted ptients who hd no complictions nd sustined more dverse hemodynmic conditions with lower stump pressure, lower vmca%, nd longer crossclmp time hd no complictions. A high level of microemboliztion 24 (14 during 18 minutes) ws found in the recovery room in ptient 211, indicting significnt thrombus source t the opertive site. Ptient 55 ws monitored for only 1 minute in the recovery room to mke determintion regrding microemboliztion. Shunted ptient 200 with high stump pressure nd high vmca% sustined trnsient worsening of the postopertive symptoms (wekness in one rm cused by previous infrction). Shunt insertion ws delyed for 58 minutes becuse of the high stump pressure. Bseline MCA velocity ws only 17 cm/sec, dropping to 16 cm/sec upon cross-clmping, presumbly due to 99% stenosis with poor collterliztion. A shunt ws eventully used becuse the surgeon worried bout these persistent low velocities. Postopertively, the operted ICA ws found to be ptent by mens of duplex Doppler exmintion. The cuse of this minor compliction ws probbly emboliztion, but the compliction lso points up previous recommendtions for lwys shunting ptients with previous infrctions. 711 Microemboliztion 24 ws detected in ll shunted nd nonshunted ptients with cerebrovsculr complictions occurring in the operting room nd ws concluded to be possible contributing cuse in ll complictions. Ptients 79 nd 38, both with poor hemodynmic prmeters but brief cross-clmp times for shunt insertion nd removl, sustined severe strokes. The primry cuse of stroke in ptient 79 ws probbly hypoperfusion, but n embolus my hve been produced when the shunt ws forced to chieve plcement. The microemboliztion monitoring dt t tht time were, however, not vilble, beyond 1 minute, to prove this. The primry cuse of stroke in ptient 38 ws concluded to be hyperperfusion occurring when MCA velocities incresed fter relese of the clmp to more thn twice the bseline vlue. A postopertive cerebrl computed tomogrm demonstrted hemorrhgic infrction in both hemispheres. The contrlterl ICA ws occluded preopertively, nd there ws nterior cerebrl rtery collterl crossover from the operted side. In ptients 267 nd 173 cerebrl complictions were clerly due to hypoperfusion. Both ptients demonstrted low MCA velocities nd low stump pressure. Ptient 267 required n 11-minute cross-clmp time due to difficulties in removing the shunt. Ptient 173 required brief cross-clmp times for the initil insertion nd removl of the shunt, but n dditionl crossclmping ws necessry when MCA velocities dropped to zero soon fter relese of the crotid clmp. Upon reopening the rtery n occluding intiml flp ws found. Fifteen minutes of dditionl cross-clmping ws required to repir the flp, during which time MCA velocities were mintined, by elevtion of the rteril pressure, t 12 cm/sec (vmca%=32). This ptient luckily sustined only trnsient postopertive wekness in the left rm. The compliction ws probbly cused by hypoperfusion during the 15 minutes of low

5 Spencer et l MCA Blood Velocity nd Stump Pressure 1443 blood flow, but the mny microemboli detected in the recovery room my hve contributed. Discussion The reltion between vmca nd stump pressure hs been reported to be liner The correltion coefficients of these studies using ptients vried from 0.46 to The liner nlyses projected to zero pressure or to n intercept on the velocity xis. Creful nlysis of these reports led to two mjor criticisms. 1) The reltions re probbly not liner becuse n intercept on the pressure xis is to be expected, requiring some criticl closing (flow cesstion) pressure so tht flow nd velocity will rech zero with some residul pressure. Moreover, ny corticl collterl circultion will tend to produce some MCA bck pressure, cusing blood flow in the MCA to be zero with some residul corticl collterl pressure. 2) The zero reference levels of the pressure mesurements were not indicted in ny of these reports nd were probbly mostly the hert level nd thus were mesured flsely high nd with gret vribility. When we eliminted, in our studies, ll uncertin zero pressure references, used only stump pressures mesured with the CCA s the guge zero reference, nd ssumed residul pressure t flow of zero nd curviliner reltion, more likely result ( pressure xis intercept) ws found. In ddition, when percentge chnge in MCA velocities rther thn the bsolute vlue ws used, the problems of using signls from n MCA brnch s well s those resulting from vrible dimeters of the MCA nd vritions in the Doppler ngle re meliorted nd the reltion is further improved. The combined effect of collterliztion nd utoregultion is inherent in the second cross-clmp pressure/velocity reltion. In those ptients with good collterl circultion nd n ctive utoregultion, disproportiontely greter drop in stump pressure ws necessry to produce perceptible chnge in MCA velocity. In ptients with poor collterl circultion, the decrese in stump pressure cused disproportionl decrese in velocity, even with norml utoregultion. The curved pressure/velocity reltion, with concvity towrd the pressure xis, demonstrtes greter relibility of vmca% over stump pressure s n indictor for selective shunting. When choosing criticl level for stump pressures (e.g., 25 or 30 mm Hg), smll overestimtion cn cuse gret underestimtion of cerebrl perfusion while errors in the vmca% mesurements re fr less criticl. This is even more importnt if stump pressures re mesured when the guge zero reference is estimted t the hert level. The proponents nd users of crotid bck (stump) pressure hve usully not indicted the zero reference level to be used. Only one rticle 11 in our review hs mentioned the need to reference the pressure zero "t the level of the crotid rtery." It should be noted tht there re no zero reference problems with Doppler-determined velocities. In ptients with lower stump pressures, rteril blood flow velocity flls precipitously with smll chnges in pressure, requiring high shunt decision level nd producing either unnecessry shunts or flse conclusions of dequte perfusion. Shunting decisions, in this series, were initilly bsed on the stump pressure, with the criterion tht shunt ws plced if the pressure fell below 30 mm Hg. After experience with TCD monitoring, greter relince ws plced on MCA velocities to the grdul exclusion of stump pressure s decision criterion. Experience with nonshunted ptients without complictions indicted tht vmca% of >40 (corresponding to stump pressure of 25 mm Hg) provides dequte brin perfusion. Archie 10 reported crefully mesured stump pressures tht re quite consistent with our findings when compring vlues uncorrected for venous pressure. Venous pressure used to clculte perfusion pressure serves vlid concept; however, venous pressure verged only 6.2±3.9 mm Hg. We believe venous pressure mesurement is minor correction to be mde considering the reltively greter importnce of rteril pressure in determining cerebrl perfusion. Archie 10 considers the mjor error in stump pressure mesurements to be incomplete occlusion of the crotid rteries, resulting in erroneously high stump pressures. This my hve occurred in some of our cses becuse no confirmtory test ws performed. This my ccount for some of the vribility of our dt but probbly ws not frequent becuse the percentge of our ptients with stump pressure of <25 mm Hg ws 20% compred with Archie's 16.2%. 10 The high correltion between stump pressure nd vmca% strongly suggests tht chnges in blood flow velocity represent chnges in volumetric flow in the MCA. This conclusion is in greement with other investigtions. 29 ' 31 The rcbf hs been poorly correlted with stump pressure in previous studies (r=0.62, 4 r=0.51, 6 nd r= ). The difference in those correltions my be due to technicl problems with rcbf mesurements or problems with pressure zero references. MCA blood flow velocity hs been compred with rcbf 8 during crotid endrterectomy. The correltion ws strong if rcbf ws <20 ml/100 g/min but wek t higher levels. The rcbf ws more specific thn vmca for EEG chnge. At cross-clmping, the rcbf/vmca reltion intercepted the rcbf xis between 5 nd 10 ml/100 g/min, indicting some corticl perfusion when vmca is zero. Also, the rcbf/stump pressure reltion 6 intersects the pressure xis between 9 nd 16 mm Hg. It ppers from these studies nd the present study tht both cerebrl perfusion nd MCA velocity cese t some positive rteril pressure between 8 nd 20 mm Hg nd tht MCA velocity my rech zero before corticl perfusion ceses. If the criticl perfusion levels below which the brin suffers irreversible dmge re ml/100 g/min, the corresponding criticl vmca% level will be In our experience no hypoperfusionrelted cerebrl complictions hve occurred with vmca% of >60. Worsening of symptoms from previous infrctions hs occurred in the vmca% rnge of 40-60, nd ll clerly hypoperfusion-relted complictions hve occurred in ptients with vmca% of <40. We believe vmca% of >40 nd stump pressure of >25 mm Hg represent vlues bove which shunting my not be necessry to prevent hypoperfusion. This belief tht vmca% of 40 represents level below which complictions re prone to occur is confirmed by recent publiction 31 compring the rtio of pre- to post-crossclmp MCA velocities in ptients with EEG flttening nd n rcbf of <20 ml/100 g/min.

6 1444 Stroke Vol 23, No 10 October 1992 The mjor problems in TCD monitoring of the MCA revolve round difficulty in finding nd holding the probe over n ultrsonic window, problems requiring continuous surveillnce of the signl. Technologist dependency is possibly not more of problem thn with monitoring rcbf or EEG, or even with surgery under locl nesthesi. The cost of equipment for TCD monitoring is modest compred with tht for rcbf nd EEG monitoring. Stump pressure nd rcbf mesurements provide only intermittent vlues nd do not continuously follow chnges throughout the procedure s does TCD. TCD offers n dditionl unique dvntge in providing on-line detection of ctive microemboliztion. 24 For those cses in which the temporl bone ultrsonic window is not vilble, lterntive use of stump pressure, or corticl function monitoring with EEG or ptient response, ppers dvisble. Where fesible, combintion of corticl perfusion nd MCA velocity monitoring techniques my be optiml. Conclusions 1. TCD monitoring of the MCA signls in ptients undergoing crotid endrterectomy provides hemodynmic dt tht cn be used to prevent hypoperfusion during crotid endrterectomy. 2. Confirmtion of pproprite monitoring of the MCA or brnch thereof cn be ssured by observtion of oscilltions in the Doppler velocity spectrum incurred during surgicl mnipultions or by direct finger oscilltions of the CCA. 3. Following cross-clmping of the CCA, the MCA blood flow velocities decrese depending on the collterl vilbility nd the utoregultion response. In cses of severe ICA stenosis or where gret collterliztion between the bsl cerebrl rteries is vilble, MCA velocities my not decrese upon cross-clmping. 4. From the first bet immeditely fter crotid cross-clmping, the cerebrovsculr resistnce progressively decreses over second period representing the utoregultion response. The first-bet stump pressure or vmca% reflects collterliztion vilble; chnges over the subsequent second intervl reflect the utoregultion response. Decision for shunting bsed on hemodynmic criteri should be mde fter the utoregultion response is complete. 5. The reltion between stump pressure nd vmca% follows defined curviliner functions with correltion coefficients of >80%. The vmca% is better index of chnge in perfusion thn is residul velocity or the bsolute difference in velocity. 6. Becuse the stump pressure/vmca% reltion is concve towrd the pressure xis, TCD velocity chnges provide greter mrgin of sfety for shunting decisions thn do stump pressure mesurements. 7. When stump pressures re mesured for shunting decisions, the zero reference level for the strin guge should be crefully set t the level of the CCA to prevent overestimtion nd subsequent dnger of hypoperfusion during crotid cross-clmping without shunting. Doppler velocity mesurements hve no zero reference mbiguities in monitoring MCA signls during crotid endrterectomies. 8. More thn 75% of ptients hve sufficient intrcrnil collterl circultion nd utoregultion response to llow cross-clmping of the crotid rteries in the neck without the use of shunt. A resonble MCA velocity below which shunting should be performed in nesthetized ptients is <40% of the bseline level. 9. MCA velocity chnges cn serve s the sole hemodynmic decision-mking prmeter, but in cses in which the Doppler signl cnnot be found stump pressure, EEG, nd other prmeters cn be used. Acknowledgments We thnk IAPM stff engineers Don Dvis nd John Klepper for ssistnce in instrumenttion. Also, we thnk the vsculr surgeons nd crdiovsculr nesthesiologists of the Providence Medicl Center in Settle, Wsh., for their enthusistic coopertion. References 1. Moore WS, Hll AD: Crotid rtery bck pressure. Arch Surg 1969;99: Moore WS, Yee JM, Hll AD: Collterl cerebrl blood pressure. Arch Surg 1973;106: Hys RJ, Levinson SA, Wylie EJ: Intropertive mesurement of crotid bck pressure s guide to opertive mngement for crotid endrterectomy. Surgery 1972;72: Boysen G: Cerebrl blood flow mesurement s sfegurd during crotid endrterectomy. Stroke 1971;2: Jennett WB, Hrper MA, Gillespie FC: Mesurement of regionl cerebrl blood-flow during crotid ligtion. Lncet 1966;2: McKy RD, Sundt TM, Michenfelder JD, Gronert GA, Messick JM, Shrbrough FW, Piepgrs DG: Internl crotid rtery stump pressure nd cerebrl blood flow during crotid endrterectomy: Modifiction by hlothne, enflurne, nd Innovr. Anesthesiology 1976;4: Hunter GC, Sieffert G, Mlone JM, Moore WS: The ccurcy of crotid bck pressure s n index for shunt requirements. Stroke 1982;13: Hlsey JH, McDowell HA, Gelmon S, Morwetz RB: Blood velocity in the middle cerebrl rtery nd regionl cerebrl blood flow during crotid endrterectomy. Stroke 1989;20: Mrshll BM, Lougheed WM: The use of electroencephlogrphic monitoring during crotid endrterectomy s n indiction for ppliction of temporry bypss. Cn JAnesth 1969;16: Archie JP: Technique nd clinicl results of crotid bck-stump pressure to determine selective shunting during crotid endrterectomy. J Vse Surg 1991;13: Quinones-Bldrich WJ, Moore WS: Intropertive monitoring nd use of the internl shunt during crotid endrterectomy. Int Surg 1984;69: Kwn JHM, Peterson GJ, Connolly JE: Stump pressure, n unrelible guide for shunting during crotid endrterectomy. Arch Surg 1980;115: Lord RSA, Moore FJ, Hill DA, Hzelton S, Horn B: Significnce of common crotid bck pressure mesurements. Surgery 1981 ;89: Lrson CP, Ehrenfeld WK, Wde JG, Wylie EJ: Jugulr venous oxygen sturtion s n index of dequcy of cerebrl oxygention. Surgery 1967;62: Pdychee TS, Gosling RG, Bishop CC, Burnnd K, Browse NL: Monitoring middle cerebrl rtery blood velocity during crotid endrterectomy. Br J Surg 1986;73: Steiger HJ, Schffler L, Boll J, Liechti S: Results of microsurgicl crotid endrterectomy: A prospective study with trnscrnil Doppler nd EEG monitoring nd elective shunting. Ad Neurochir (Wien) 1989;100: Thiel A, Russ W, Zeiler D, Dpper F, Hemplemnn G: Trnscrnil Doppler sonogrphy nd somtosensory evoked potentils monitoring in crotid surgery. EurJ Vse Surg 1990;4: Ackerstff RGA, Jnsen C, Vriens EM, Eikelboom BC, Vermeulen FEE: Cerebrl function monitoring during crotid endrterectomy by simultneous electroencephlogrphy nd trnscrnil Doppler sonogrphy. (bstrct) / Crdiovsc Tech 1990;9: Sollmnn WP, Lorenz M, Gb MR, Dorfmuller G, Hinrichs H, Feistner H: Intropertive monitoring with TCD, EEG nd evoked potentils in extrcrnil cerebrovsculr surgery, (bstrct) J Crdiovsc Tech 1989;8:17175

7 Spencer et l MCA Blood Velocity nd Stump Pressure Hlsey JH, McDowell HA, Gelmon S: Trnscrnil Doppler nd rcbf compred in crotid endrterectomy. Stroke 1986;17: Powers AD, Smith RR, Greber MC: Trnscrnil Doppler monitoring of cerebrl flow velocities during surgicl occlusion of the crotid rtery. Neurosurgery 1989;25: Bss A, Krupski WC, Schneider PA, Otis SM, Dilley RB, Bernstein EF: Intropertive trnscrnil Doppler: Limittions of the method. J Vse Surg 1989;10: Jorgensen L, Schroeder T, Knudsen L, Perko M: Trnscrnil Doppler monitoring nd crotid rtery blood pressure mesurements during crotid endrterectomy. (bstrct) / Crdiovsc Tech 1990;9: Spencer MP, Thoms GI, Nicholls SC, Suvge LR: Detection of middle cerebrl rtery emboli during crotid endrterectomy using trnscrnil Doppler ultrsonogrphy. Stroke 1990;21: Edelmnn R, Ringelstein EB, Richert F: Trnscrnil Doppler sonogrphy for monitoring the middle cerebrl rtery blood flow velocity during crotid endrterectomy. Rev Brs Angiol Ore Vse 1986;16: Bernstein EF: Role of trnscrnil Doppler in crotid surgery: Noninvsive dignosis of vsculr diseses. Surg Clin North Am 1990;70: Crson SAA, Thoms GI, Cstillos F, Aslid R, Powell M, Spencer MP: Trnscrnil middle cerebrl rtery Doppler monitoring during crotid endrterectomy. (bstrct) J Crdiovsc Tech 1987;7: Spencer MP, Grndo L, Aslid R, Thoms G, Crson SA: Reltionship between MCA pressure nd velocity during crotid endrterectomy. (bstrct) / Crdiovsc Tech 1989;8: Lindegrd K-F, Lundr T, Wiberg J, Sj0berg D, Aslid R, Nornes H: Vritions in middle cerebrl rtery blood flow investigted with noninvsive trnscrnil blood velocity mesurements. Stroke 1987;18: Spencer MP: Detection of cerebrl rteril emboli, in Newell DW, Aslid R (eds): Trnscrnil Doppler. New York, Rven Press, Ltd, 1992, ch 19, pp Jorgensen LG, Schroeder TV: Trnscrnil Doppler for detection of cerebrl ischemi during crotid endrterectomy. Eur J Vse Surg 1992;6: